This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. Primary support for the subproject and the subproject's principal investigator may have been provided by other sources, including other NIH sources. The Total Cost listed for the subproject likely represents the estimated amount of Center infrastructure utilized by the subproject, not direct funding provided by the NCRR grant to the subproject or subproject staff. It has been known for some time that during muscle contraction the long N terminus of cardiac ELC makes direct contacts with actin. To study how this might regulate contractility, we generated transgenic mice expressing a 43 amino acid residue truncation of the ELC N terminus (Tg-D43) (Kazmierczak et al. JMB 387, 706-725, 2009). We hypothesize that the N-terminal ELC extension regulates force development in cardiac muscle via its direct contacts with actin and/or through influencing the conformation of the myosin heavy chain(MHC) and its interaction with actin,. Another possibility is that the N-terminus of ELC acts as a tether pre-positioning the myosin heads for attachment to actin and facilitating Acto-myosin interaction. The lack of the N-terminal ELC extension would therefore result in a decreased maximal force and lower myofilament cooperativity, the phenotypes that we observe in our published preliminary data with Tg-D43 mice. This project will help determine whether the genetic ablation of the N-terminus of ELC may result in radial and/or azimuthal displacement of Tg-D43 crossbridges, such that they may re-locate further from the surface of the thin filaments and closer to the myosin thick filaments. To test this, we will use low-angle X-ray diffraction to compare lattice spacings and equatorial intensity ratios (I1,1/I1,0) to assess the proximity of myosin cross-bridge mass relative to actin in Tg-D43 mouse myocardium compared to Tg-WT mice containing a full length ELC N-terminus. This will elucidate the regulatory role of the N-terminal ELC extension in muscle contraction.